This invention relates generally to printed circuit boards and manufacturing methods and particularly to printed circuit boards adapted to carry signals having very high data rates.
As is known in the art, printed circuit boards are used to couple data among electrical components mounted on a surface of such board. This data is coupled through signal conductors disposed at various levels in the board. The board also carries reference potentials, such as ground and component supply voltages. In order to provide isolation between signal conductors in the various levels, reference potential conductive planes are interspersed between the levels of signal conductors.
As is also known in the art, in order to connect to the components, conductive vias are provided which pass from the surface of the board to the interior region thereof for connection to the signal conductors. These vias form what are sometimes called "signal launches." Further, in many cases it is necessary to electrically connect components on one printed circuit board to components on another printed circuit board. This is typically done with an electric connector assembly. The connector assembly has two parts, each part mounted to a corresponding one of the printed circuit boards. Thus, when it is desired to connect one board to the other board, the connector parts are plugged into one another.
Further, as is known in the art, the data rates of the signals propagating through printed circuit boards are ever increasing. As such data rates increase, it is necessary to improve the efficiency of the electrical coupling of the data passing through one of the printed circuit boards to the other printed circuit board. More particularly, the signal conductor, reference potential plane or layer, and portions of the dielectric of the board therebetween form a transmission line having a predetermined impedance, typically 50 ohms. The effect of the via provides an impedance mismatch to the impedance of the transmission line. One technique used to compensate for this impedance mismatch is to provide compensation elements in the connector. For vias with impedance below the impedance of signal traces in the boards, some connectors are designed to have a higher impedance so that on average the impedance is the same as the interconnect as it is in the board. However, signal reflections which can limit performance of an interconnect depend on changes in impedance.